Dan, you didn’t say if the cold water was affected at the same faucet outlets. If there is also no cold water at the faucets, try removing the aerators and check for flow. There may also be a problem with the cold water supply valve to the water heater, make certain that valve is totally open. Open and close it several times to make ceretain that scale formations have not accumulated on the face of the valve seat. Before you agree to pay $5000 for a repipe, get another professional plumber in to look at the problem.